colorpickershell: Unpack the tuple returned from PickColor()

colorpickershell: Unpack the tuple returned from PickColor()

When calling PickColor on org.gnome.Shell, we get back an "a{sv}", which
GDBus provides to us as "(a{sv})".

At the minute we're not unpacking this tuple, and so picking fails with
messages like:

  GLib-CRITICAL **: 13:38:19.439: g_variant_lookup_value: assertion 'g_variant_is_of_type (dictionary, G_VARIANT_TYPE ("a{s*}")) || g_variant_is_of_type (dictionary, G_VARIANT_TYPE ("a{o*}"))' failed

  Gtk-WARNING **: 13:38:19.439: Picking color failed: No color received

Let's unpack it.